The present invention concerns a mass spectrometry and, particularly, it relates to a mass spectrometer that can be utilized, for example, for the detection of contaminants such as residual agricultural chemicals present by slight amount in atmospheric air, drinking water, foods, etc. or detection of dangerous matters.
Mass spectrometry is an essential technique as means for identifying substances and has been utilized generally for application uses such as detection of environmental food contaminants present by slight amount in atmospheric air, drinking water, foods, etc., or detection of dangerous matters.
In mass spectrometry, molecular samples separated from specimen by using an appropriate pretreatment device such as a gas chromatograph are introduced in vacuum and ionized and created as sample ions. The sample ions are identified by measuring for the ratio of charge and mass (charge-mass ratio) by using electromagnetic fields in amass spectrometric section. Various kinds of pretreatment devices, ions sources and mass spectrometers have been known and identification of substances is conducted in a wide range of fields by appropriately selecting and combining them while utilizing the feature coping with the object to be analyzed.
The chemical ionization includes, as is well-known, positive chemical ionization for creating positive ions and negative chemical ionization for creating negative ions. In many cases, a sample gas separated by a pretreatment device such as a gas chromatograph is introduced in vacuum, ions are created by chemical ionizing reaction and they are analyzed by an ion trap mass spectrometer, etc.
In the positive chemical ionization, when an electron e− at high energy (about 70 eV) is irradiated to molecule species G referred to as a reagent gas, a primary positive ion G+ is created by the process of electron impact between the reagent gas and the electron (electron impact ionizing reaction of formula (1)) and a positive sample ion M+ is created by the transfer of a positive charge from the primary positive ion G+ to the sample gas molecule M (charge transfer reaction of formula (2)). The created positive sample ion M+ is subjected to mass analysis is identified. In the positive chemical ionization, methane (CH4), etc. are utilized generally as the reagent gas of molecule species G.G+e−(several tens eV)→G++2e−  (1)G++M→G+M+  (2)
On the other hand, in the negative chemical ionization, when an electron e− at low speed (1 eV or lower) is irradiated to the reagent gas of molecule species G, a primary negative ion G− ions are created by the electron capture process by the reagent gas molecule species G (electron capture reaction of formula (3)) and a negative sample ion M− is created by electron donation and reception between the created primary negative ion G− and the sample molecule M (charge transfer reaction of formula (4)). In the negative chemical ionization, molecule of water, etc. are utilized generally as the reagent gas molecule species G.G+e−(1 eV or lower)→G−  (3)G++M→G+M+  (4)
In a case of conducting creation of the sample ions and the mass analysis of the sample ions in different places, it requires an ion transportation section for ion transportation between the ion source section and the mass spectrometric section. This results in loss of the sample ions caused by ion transportation. On the other hand, in a case of conducting creation of the sample ions and mass analysis of the sample ions in one identical place, that is, in a mass spectrometer in which the ion source section and the mass spectrometric section are identical, loss of the sample ions is not caused, and mass spectrometry at high efficiency is possible.
A mass spectrometer having an ion source section for conducting positive chemical ionization is well-known (for example, refer to Japanese Patent Application Laid-Open No. 6-96727). The positive chemical ionization is often conducted inside the ion trap.
FIG. 9 is a view for explaining the outline of a mass spectrometer in a prior art where an ion source section for conducting positive chemical ionization and a mass spectrometric section are used in common.
A three dimensional ion trap comprises a ring electrode 201, and two end cap electrodes 202 and 203. When a reagent gas used for positive chemical ionization (flow of reagent gas is shown by arrow 205) and a sample gas containing a sample molecules separated by gas chromatograph (GC) 206 (flow of sample gas is shown by arrow 207) are introduced to a vacuum vessel where the three dimensional ion trap is placed. Thermal electrons generated by a tungsten filament 204 are accelerated and have a kinetic energy at about 70 eV. The accelerated thermal electrons are introduced as an electron beam 208 to the inside of the three dimensional ion trap.
G+ is created by the reaction of formula (1) between the electron and the reagent gas molecule G introduced to the inside of the three dimensional ion trap and the created G+ is reacted with the specimen gas molecule M (formula (2)) to create the sample ion M+ 209. The created sample ion M+ 209 is ejected mass selectively from the three dimensional ion trap by using a well-known three dimensional ion trap mass spectrometry and detected by an ion detector 210. The sample molecule is identified based on mass spectra.
A mass spectrometer having an ion source section for conducting negative chemical ionization is well-known (for example, refer to Japanese Patent Application Laid-Open No. 9-306419). In the prior art, creation of negative sample ions and separative detection of the sample ions are conducted generally in different places, that is, the ion source section and the mass spectrometric section are placed independently of each other. The ion source section is often referred to as an external ion source. Negative sample ions created by the external ion source are introduced into the mass spectrometric section. In the prior art, creation of the sample ion by the negative chemical ionization in the ion trap has been utilized only limitatively such as for scientific researches since the reaction efficiency of the formula (3) is low.
FIG. 10 is a view for explaining the outline of a mass spectrometer in the prior art having an external ion source section for conducting negative chemical ionization.
As shown in FIG. 10, an ion trap mass spectrometric section in which a three dimensional ion trap comprising a ring electrode 201 and two end cap electrodes 202 and 203 is placed, and an external ion source vessel 301 for conducting negative ionization of a sample are placed independently of each other. A reagent gas used for negative chemical ionization (flow of reagent gas is shown by arrow 205) and a gas containing a sample separated by a gas chromatograph 206 (gas flow containing sample is shown by arrow 207) are introduced into an external ion source 301. Thermal electrons generated by a tungsten filament 204 are introduced as an electron beam 208 at a low energy to the external ion source vessel 301.
G− is created by the reaction between the electron and the reagent gas molecule G introduced to the inside of the external ion source vessel 301 (formula 3) and the created G− is reacted with the sample gas molecule M (formula 4) to create a sample ion M− 302. The created sample ion. M− 302 is moved from the external ion source vessel 301 through diffusion to the three dimensional ion trap as shown by a horizontal arrow shown in FIG. 10. The sample ion M− 302 is ejected mass selectively from the three dimensional ion trap by using a well-known dimensional ion trap mass spectrometry and detected by an ion detector 210. The sample molecule is identified based on the detection signal by the ion detector 210.
A linear ion trap mass spectrometric system is well-known (refer, for example, to the specification of U.S. Pat. No. 5,420,425 and the specification of U.S. Pat. No. 6,177,668), the technique regarding a linear ion trap axial resonance ejection on the linear ion trap mass spectrometry is well-known (refer, for example, to the specification of U.S. Pat. No. 5,783,824). An ion guiding technique using linear RF generating multipole electrodes not in parallel is well-known (refer, for example, to the specification of U.S. Pat. No. 5,847,386).